Size reduction machines of the prior art utilise a frusto-conical shaped screen located in a channel between an input and an output. Such size reduction machines are more particularly described in U.S. Pat. No. 4,759,507. In these machines, various screens and impellers are used depending on the size and type of product that is being processed. The screens have apertures in different sizes and shapes to produce a desired milled product.
Once a screen and impeller have been selected, the operation and efficiency of the machine depends upon the gap between the impeller and the interior wall surface of the screen. The different wall thicknesses of the screen are compensated for by inserting or removing spacers on the impeller shaft to move the impeller relative to the interior wall surface of the screen. Since the wall of the screen is tapered relative to the impeller, the actual adjustment of the gap is less than the thickness of the spacer and depends upon the angle of the screen relative to the horizontal. Where the tapered wall of the screen has an angle of sixty degrees relative to the horizontal, the gap is adjusted by one half the thickness of the spacer.
The adjustment of the gap becomes more complicated when it is desired to use a new screen having a different wall thickness and at the same time varying the gap size. It is thus necessary to remove or insert spacers on the impeller shaft whenever a screen having a different wall thickness from the previous screen is used in the machine.
For each set of screen and impellers, a variety of spacers must be provided with the machine. In order to arrive at the proper spacing between the impeller and the screen, the impeller must be installed with a first spacer. If the impeller rubs against the screen, the impeller must be removed to remove the first spacer and replace it with a second incrementally smaller spacer. The process is repeated until there is no metal to metal contact between the impeller and the screen.
Alternatively, if the impeller does not rub against the screen, the steps are repeated with incrementally thicker spacers. The process is repeated until contact is heard whereupon the next incrementally smaller spacer replaces the previous spacer establishing the proper gap setting.
The gap between the impeller and the screen is critical for producing a final milled product of consistent particle size. If the gap is too large, there is a loss of capacity or throughput, screen binding and a change in particle size. If no gap exists between the impeller and the screen, the screen and the impeller will become worn or burned and in the extreme, the impeller will not rotate.
The use of spacers is mandatory to produce satisfactory results for consistent particle size of milled product. However, the calibration process of installing a spacer and repeatedly removing and replacing incremental spacers is time consuming. Further, since the spacers must be incrementally sized and machined, the cost of producing such spacers is relatively high. Spacers are easily lost during cleaning which can lead to re-assembly of the size reduction machine with an improper gap setting and decreased performance.
When more than one spacer is used to achieve the proper gap setting, narrow gaps or machine crevices are created. These gaps or crevices are to be avoided in sanitary applications of the size reduction machine.
Maintaining the gap between the impeller and the screen is imperative for maintaining a consistent particle size of the milled product. Therefore it is essential that the impeller be fixed relative to the screen during operation. Spacers have been found to be well suited for this application since the spacer will not measurable vary during operation of the size reduction machine.
Adjustable spacer means have been proposed to replace the spacers. Such a device is illustrated in U.S. Pat. No. 4,759,507. The apparatus of the prior art mounts the spindle within a housing which threadably engages the machine housing. By rotating the spindle housing relative to the machine housing, the spindle and the impeller move relative to the screen for adjustment of the gap. However, since the spindle receives the drive for rotation thereof via a series of belts and pulleys, relative displacement of the spindle requires relative displacement of the drive motor to maintain alignment between the drive motor and the pulley mounted on the spindle. The additional adjustment not only increases the time for calibrating the apparatus and ultimately the cost for designing and using the apparatus, different personnel may be required to undertake the adjustment of the drive motor.
The use of threads to provide a method of adjusting the gap has traditionally been unacceptable for use in a size reduction machine. Size reduction machines are widely used in sanitary environments. For example, the production of pharmaceuticals and cosmetics require very strict sanitary standards for operation and production. It is difficult to adequately clean between the threads if the threaded portion is not removable from the machine. Accordingly, the pharmaceutical industry has rejected any machines which are not capable of being fully sanitized.